ep - p h / 99 08 46 6 v 1 2 5 A ug 1 99 9 1 Single target - spin asymmetries in semi - inclusive pion electroproduction on longitudinally polarized protons ∗

We evaluate the single target-spin sin φ h and sin 2φ h azimuthal asymmetries in the semi-inclusive deep inelastic lepton scattering off longitudinally polarized proton target under HER-MES kinematic conditions. A good agreement with the HERMES data can be achieved using only the twist-2 distribution and fragmentation functions. Significant single-spin asymmetries have been observed in experiments with transversely polarized proton and anti-proton beams [1]. Recently new experimental results on az-imuthal asymmetries became available. Specifically, the first measurements of single target-spin azimuthal asymmetries of pion production in semi-inclusive deep inelastic scattering (SIDIS) of leptons off a longitudinally polarized target at HERMES [2] and off a transversely polarized target at SMC [3], and the observation of the azimuthal correlations for particles produced from opposite jets in Z decay at DELPHI [4]. In this note we present estimates of the single spin azimuthal asymmetry in the SIDIS on a longitudinally polarized nucleon target for the HERMES kinematic conditions. Our approach is based on the parton model description of polarized SIDIS [5]. The cross-section contains the (1/Q) 0-order terms coming from leading dynamical twist-two distribution and fragmentation functions (DF's and FF's) as well as (1/Q)-order kinematic twist-three terms arising due to the intrinsic transverse momentum of the quark in the nucleon. We will neglect the (1/Q)-order contributions of the higher twist DF's and FF's obtained in [6]. Thus, our approach is similar to that of [7] in describing the cos φ h asymmetry in unpolarized SIDIS. Let k 1 (k 2) be the initial (final) momentum of the incoming (outgoing) charged lepton, Q 2 = −q 2 , q = k 1 − k 2 – the momentum of the virtual photon, P and P h (M and M h) – the target and final hadron momentum (mass), x = q 2 /2(P q), y = (P q)/(P k 1), z = (P P h)/(P q), P hT (k 1T) – the hadron (lepton) transverse with respect to virtual photon momentum direction and φ h – the azimuthal angle between P hT and k 1T around the virtual photon direction. Note that the azimuthal angle of the transverse (with respect to the virtual photon) component of the target polarization, φ S , is equal to 0 (π) for the * talk presented by K. Oganessyan at the Workshop on the structure ot the Nucleon (N